Many of the world's most famous buildings are built of oolitic limestone, often from the Jurassic era. Yet despite its widespread application, geologists disagree on how this important rock formed. Now, a new theory has provided a model whereby microbial communities created the building blocks from which this limestone is made, solving many of the problems with previous theories.

Since before Roman times, people have been fascinated by the tiny calcite or aragonite spheres (or ooids) that make up oolitic limestone. Professor Bob Burne of the Australian National University told IFLScience that they were once explained as being fossilized fish roe, and indeed the name comes from the Greek word for egg. However, ooids have been found dating back 2.7 billion years, making them far older than any fish, or indeed animals of any form.

The dominant theory today is the snowball model. It proposes small items were rolled back and forth by wind and waves in shallow tropical seas, accumulating material to form spherical shapes.

Burne can barely contain his contempt for this idea. For one thing, he pointed out to IFLScience, “there is no such thing as a snowball in nature. Also, if you do make a snowball, it won't be perfectly spherical.”

A limestone rock formed from encrusted ooids. The spaces between make it light but strong, yet the formation of the ooids has been a mystery. Lannon Harley/ANU

Burne has co-authored a Scientific Reports paper mathematically modeling the formation of ooids by microbial biofilms a layer at a time.

In a startling example of how distant scientific fields benefit each other, the work was based on H.P. Greenspan's groundbreaking explanation of a type of brain tumor growth.

Burne's biofilms, made of diverse species of microbes, wrap around a nucleus and absorb surrounding nutrients. Mineralization occurs when microbes in the inner layers die. Ooids stop growing when they run out of nutrients in the immediate surroundings. Although ooids are usually small (sometimes defined as being less than 2 millimeters/0.08 inches across), rare conditions of nutritional abundance allow them to keep growing. In the aftermath of the Permian extinction, 5-centimeter (2-inch) ooids were formed, which Burne attributes to the lack of competition for nutrients.

Under a microscope, the layered nature of ooids is visible. ANU

Precipitated calcium carbonate cements ooids together to form the limestone that has gone into buildings as famous as the British Museum, St Paul's Cathedral, Buckingham Palace, and parts of the Pentagon. The spaces between the spherical ooids make the rock relatively light but strong, and therefore perfect for building, as well as holding reservoirs of oil or water.

Burne doesn't expect an understanding of its formation to change how we use limestone or lead to any technological breakthroughs. However, it may alter the way we look at the world a fraction. "Our research has highlighted yet another vital role that microbes play on Earth and in our lives," Burne said in a statement.

A comparison of an ooid from the Triassic in China and the layers predicted by Burne's mathematical model. Batchelor et al/Scientific Reports